When my son started skating we went to "Play it Again Sport" to buy him skates. My husband who has a lot of hockey playing and coaching experience said that at such a young age when childrens' feet grow so quickly second-hand skates are not a problem. Once their growing slows down, you should invest in a new pair because it will be able to be moulded to only your child's foot. Helmets should always be purchased new since you want to make sure the helmet has never been in a collision or serious injury.
I would also recommend buying shin pads. That way, if your 4- year old falls, his knees and legs will have some cushion. My son wasn't afraid to fall once he had them on and they will fall a lot in the beginning!
Happy skating!